Piezoelectric crystal mounting



y 13, 1939- s. A. BOKOVOY 2,166,713

PIEZOELECTRIC CRYSTAL MOUNTING Filed March 30, 1935 clampzln pres-SureosczlZZa tin, dimension maximum movement t r of crystal zero movemeV ofcryatal l M INVENTOR:

Samuel ALBo/iovoy WMM BY HTTOHNEY Witness Patented July 18, 1939 UNITEDSTATES PATENT OFFICE Samuel A. Bokovoy, Audubon, N. J., assignor toRadio Corporation of America, a corporation of Delaware ApplicationMarch 30, 1935, Serial No. 13,887

Claims.

This invention relates to piezoelectric crystal apparatus, particularlyto electrode plates for quartz bar or width-oscillating crystals, andhas special reference to the provision of an improved 5 electrodeassembly for so-called air-gap, pressure, node-mounting of suchcrystals.

Numerous mounting arrangements for piezoelectric crystals are known inthe art and various advantages are claimed for each type mounting. Thusthe gravity or simple air-gap holder, wherein the crystal is simply laidwithout clamping restriction on the inner bottom surface, or on a riser,within a closed container, is said to possess the advantage of ensuringsubstantial constant frequency of oscillation. This desirable characteristic, however, can only obtain when the box is highly insulated fromshocks or tremors tending to dislocate the crystal. Such mountings areobviously entirely unsuited for use in moving vehicles and in otherinstallations where the crystal and its electrodes might be subject torelative movement.

Another type mounting provides a pair of spaced, fiat surfaced,electrodes with the oscillator clamped in face to face relationtherebetween. Such mounting ensures rigidity but at a considerablesacrifice of frequency-accuracy and amplitude of oscillation. Thesedisadvantages are only slightly less in degree when the crystal ismounted at a plurality of distributed points, especially so if. any ofthe supporting points are adjacent the area of maximum movement in theoscillatory dimension.

Where, as is usually the case, freedom from mechanical displacement andfreedom of oscillatory movement are prime desideratum it has beenproposed to support the crystal adjacent its nodal axis and to apply theclamping pressure, which is necessary to secure the crystal againstdisplacement, through the quartz center. One such known arrangementrequires the formation of a detent in the surface of the crystal,another employs an adjustable screw mechanism, still another requiresthat the crystal be electroplated; all such node mounting arrangementsare expensive and some are quite complicated.

The recent development of crystals having a substantially zerotemperature coefficient of frequency has made practical the use ofpiezoelectric elements in fields which, up to now, have been closedbecause of the high cost of the necessary thermostatically controlledcasings and associated frequency-fixing apparatus. With old typecrystals the cost of the electrodes per se comprised only a smallpercent of the total cost of the apparatus so that the manufacturer wasjustified in employing plates and mountings of the above describedexpensive construction. Similar plates used with modern crystals would,however, constitute a major factor in the total cost of the apparatusand render uneconomical the application of crystals in those fieldsWhere cost is a prime consideration.

Accordingly, principal objects of the invention are to provide aninexpensive, simple and trouble-free nodal zone mounting of maximumrigidity and minimum contact for piezoelectric crystals and further toensure maximum output efficiency for such crystals.

Other objects will be apparent and the invention itself will be bestunderstood by reference to the following description taken in connectionwith the accompanying drawing wherein:

Figure 1 is a perspective view of one. of a pair of electrode plateswithin the invention and shows oppositely located risers adapted tosupport a crystal at spaced points within its nodal zone;

Fig. 2 is a modification within the invention showing an electrode platehaving a support riser along an axis corresponding to the nodal axis ofa crystal;

Fig. 3 is an electrode plate for supporting a crystal adjacent itsmechanical center;

Fig. 4 is a plan view of, a modification of the plate of Fig. 3;

Fig. 5 is a vertical section taken on the line 5---5 of Fig. 4 andindicates one method of forming a riser;

Fig. 6 is a diagram of a crystal indicating certain areas of stress, ofinterest in connection with the invention;

Fig. 7 is a side elevation of a dustproof casing containing apiezoelectric crystal mounted be tween electrode plates of the presentinvention, the casing itself forming no part of the invention; and

Fig. 8 is a sectional View taken on the line 8-8 of Fi 1.

While the invention will be explained in connection with a quartzcrystal cut to oscillate along one of its greater dimensions, it is tobe understood that the electrode plates of the present invention may beemployed in the mounting of so-called thickness oscillators and when soused will permit substantially complete freedom of, oscillation.

In a piece of quartz, designed to oscillate along one of its greaterdimensions, the movement is greatest adjacent the terminal ends of theoscillating dimension and standing waves may be said to be presentapproximately midway of the crystal along an axis parallel to theseends, as represented in Fig. 6. The electrode plates of Figs. 1 to 5inclusive are designed to contact the crystal along this nodal axis, orline of zero movement. While theoretically this axis is a mere line, forall practical purposes there is a small area or nodal zone against whichclamping pressure may be applied without appreciable damping effect uponthe crystal.

Referring now to Fig. l, which shows a preferred embodiment of theinvention. The electrode of this figure comprises a rectangular wafer orplate l l, formed of brass or other metal. It is pref erablysubstantially coextensive in length and breadth with the crystalassociated therewith; it may be of wafer thickness but should besufficiently rigid to obviate prolonged parasitic oscillations ifjolted. The major face a, a of plate ii is flat, and there are a pair ofsharply defined, inwardly directed, oppositely located, wedge shapedrisers l2-l2' thereon. The axis of symmetry s--s of these risers l2 l2'and of the plate H itself, corresponds to the previously described nodalaxis of the crystal to be supported thereon.

A convenient way of manufacturing the plates of Fig. 1 in quantity, isto mark off a number of roperly oriented circles on a sheet of theelectrode material, cut out the rectangular blanks and machine thesurface of the blanks, following the circular markings, to the desireddepth. The top surface of risers lZ-I2' should ordinarily extend .000'75to .002" above the major surface of the plate but for specialapplications may projectfrom .00025" to say .02.

The electrode plate 20 of Fig. 2 has a plurality of rectangular risers22, 22a, 22b spanning the entire width or length, as the case may be, ofthe plate so that the crystal may be positively clamped at spaced pointsalong its entire nodal axis.

The crystal contacting riser 32 of Fig. 3 is disposed at the center ofthe plane of the surface of the plate 30 which center corresponds to apoint midway of the nodal axis ss of the crystal. The riser shown iscircular but may be of any other convenient shape.

In Fg. 4 two risers 42 and 42 are shown, oppositely located on the axisof symmetry s-s of electrode plate 40.

Fig. 5 shows how, instead of cutting away the surface of the main bodyof the plate as described in connection with Fig. 1, the plates may besimply formed from sheet metal by pressing or punching with a tool ofthe desired shape on the reverse side b of the metal, so that a sectionon the major surface a will protrude. The surface of the protrudingmetal may then be ground, if necessary, to the exact height and flatnessrequired.

Fig. 7 shows a crystal i clamped by means of a spring clamp 2 betweentwo electrode plates I l--l l, the plates of this embodiment beingsimilar to that shown in detail in Fig. 1. The opposite major faces ofthe crystal I each contact only that portion of the adjacent plateconstituted by risers l2 and I2 so that there is an.

air-gap separating the crystal from each plate except for the small areawithin the nodal zone of the crystal embraced by the risers. Since, asabove set forth, risers l2-I2 are positioned within the nodal zone ofthe crystal, the pressure exerted by spring 12 and the weight of theupper electrode plate II in no way damp its natural period ofoscillation.

The entire assembly is preferably enclosed within a dustproof box orcasing 5.

As a number of possible embodiments may be made of the above invention,and as changes may be made in the embodiments set forth, withoutdeparting from the spirit or scope of the invention, it is to beunderstood that the foregoing is to be interpreted as illustrative andnot in a limiting sense except as required by the appended claims and bythe prior art.

What is claimed is:

1; An electrode for piezo-electric crystals comprising a substantiallyflat metal plate having a pair of sharply defined, flat surfaced, wedgeshaped, crystal supporting risers thereon, the apex of each wedge beinginwardly directed from opposite edges of said plate and defining an axis5 crystal supporting risers thereon, said sections.

being positioned on axis of symmetry of said plate.

3. An electrode in accordance with claim 2 and wherein said risers arecircular in shape.

4. An electrode in accordance with claim 2 and wherein said risers arerectangular in shape.

5. The combination with a piezo-electric crystal element of a pair ofmetal electrode plates, each plate having an axis of symmetry coincidingwith a nodal axis of said element and 'hav-- ing a well defined raisedportion raised .00025 inch to .02 inch from its main surface on saidaxis of symmetry, said raised portion contacting a crystal face of saidelement along only a portion of its said nodal axis, and means forclamp;

ing said element between said plates.

SAMUEL -A. BOKOVOY.

